2The problem with an answer that is entirely a quote--especially in a case like this--is that it means the question is "general reference." This answer comes from the second google hit for "por vs para" – Flimzy – 2011-11-16T00:42:55.453

4+1 for "La Bamba"! But -1 for an answer that is nothing more than copy-paste. – Richard – 2011-11-15T21:32:33.947

"Yo trabajo por ti" may also mean "I'm working for you" and "I'm working because of you" – skan – 2016-12-26T02:10:56.570

To add to @skan comment "yo trabajo por ti" could mean "for you", "because of you" or "instead of you". Also I think that "tu me llamas por teléfono" should have "tu <- teléfono" in front to keep with your logic of subject<-object/complement. Am I right Flimzy? (Good answer +1) – DGaleano – 2016-12-26T18:19:08.417

3

I know this is old but I'd like to add my answer as well. The lists have been taken from a grammar textbook but most examples (except the last two) are not just pasted... sometimes I needed some help with the structure or ideas, though. :) So if you spot a mistake, feel free to correct.

Por

Causa (cause)

Todo sucedió por ti. — Everything happened because of you.

Complemento de agente de la voz pasiva (it indicates the complement in the passive)

La carta fue escrita por Juan. — The letter was written by Juan

Localización aproximada (rough localization)

La casa estaba por aquí. — The house was around here.

Movimiento a través de un lugar (movement through a place)

El hombre pasea por el parque. — The man goes for a walk in the park.

Periodicidad (regular recurrence)

Voy a las clases cinco veces por semana. — I attend the classes five times a week.

@user Apparently, someone thinks that having their blood spilled is doing a favor to all men for the purpose of providing forgiveness "para (implied: proporcionar) el perdón" for their alleged "sins." – nachocab – 2016-11-02T14:25:44.000

2

To put the Por or Para question into something easier to remember, I have put the reasons (taken from Puntos de Partida Spanish Textbook) in a visual style similar to Flimzy's arrows of intention, but with a few additions on the Por side.

Por is represented by a backwards arrow to represent the following meanings:

Because of,
Due to

Por is represented by a circle with an arrow running through it for the following meanings:

Duration, (length of time pictured by the part of the arrow inside the circle)
By means of,
Through

Por is represented by a recycle symbol for the following meanings:

In exchange for,
In place of

Para is represented by a forward pointing arrow:

In order to,
To be used for,
To be given to,
Future deadline,
In the employ of,
Towards

(One meaning of para that doesn't fit with the forward arrow is "in comparison with others in the group." I don't have a picture for that one.

If anyone knows how to put visuals in here to help with this explanation, please let me know. It would look better with the circles and arrows actually drawn in. I'm very new to this.

1You can do the diagram in the tool of your choice, put the image somewhere public (dropbox folder, imgur, any image hosting site) and then link to it from here. – Eric Burcham – 2012-02-27T17:36:15.590